The document summarizes stellar processes involved in star formation and evolution. It describes how molecular clouds collapse under gravity to form protostars, which grow in temperature and luminosity to become main sequence stars fueled by nuclear fusion. It discusses how stars of different masses evolve after exhausting their hydrogen, either expanding into giants or supergiants and fusing heavier elements. For very massive stars, the core may collapse in a supernova that ejects most of the star's mass and produces neutron stars or black holes.

1. Astro - HL
D4 stellar processes

2. The Perfect Storm
A small region in the Swan Nebula, 5,500 ly away, described
as 'a bubbly ocean of hydrogen and small amounts of oxygen,
sulphur and other elements'.

3. Star Formation - recap
Gravity begins to pull the gas and dust
together.
They lose gravitational potential energy…
this is converted into kinetic energy.
The temperature increases.

5. Jeans criterion
In order for stars to form a portion of the molecular
cloud of particles must first collapse to become the
kernel. This gravitational collapse only occurs in
certain situations.
James Jeans showed that a cloud of given radius
and temperature, has a critical mass (the Jeans
mass) which if exceeded, will cause the cloud to
become unstable and collapse.

6. Jeans mass
The Jeans mass depends on the
radius of the cloud,
temperature,
average mass of the particles in the cloud
MJ = 3kTR / 2Gm

7. Star Formation
Protostar
High temperature leads to ionisation of elements.
E-M energy is emitted.
The star can have considerable Luminosity, eg 5000
times the surface area and 100 times as Luminous
as our Sun.
Temperature continues to increase…
Electrons stripped from the atoms in the core.
A plasma is formed.

8. Star Formation
Main Sequence Star-
Nuclear Fusion starts up.
Temperatures now high enough to fuse Hydrogen
into Helium.
Gravitational contraction will now stop as the
Fusion process will offset the contraction.
“Hydrostatic Equilibrium”

9. You are not
Required to
Memorize these
Reactions

11. Leaving the main sequence
A main sequence star has THREE main layers:
 Helium core (inner layer): Releases energy as it
contracts.
Fusion shell: Releases energy as it fuses hydrogen into
helium.
Hydrogen envelope (outer layer): Absorbs energy, and
expands greatly in size.
These swollen stars, no longer on the main sequence, are
now
giants (if M < 8 Msun) or supergiants (if M > 8 Msun).

12. stars with M > 0.4 Msun
 Can fuse helium into carbon by the triple alpha process.
This fusion process combines three helium nuclei, or
alpha particles, into one carbon nucleus.
Step one:
4
He + 4
He --> 8
Be
Step two:
8
Be + 4
He --> 12
C + �
Optional additional reaction:
12
C + 4
He --> 16
O + �

13. Stars with M > 4 Msun
Now the giants have FOUR main layers:
 Carbon (+oxygen) core: Release energy as
it contracts.
Helium fusion shell: Releases energy by
fusing helium into carbon (+oxygen)
Hydrogen fusion shell: Releases energy by
fusing hydrogen into helium
Hydrogen envelope: Still has very large
radius

14. Once the central temperature T >
600,000,000 Kelvin.
Carbon & oxygen can fuse into heavier
elements, such as silicon, sulfur, and iron –
for the star this is a new energy source.
Iron is the end of the line.

15. Mass – Luminosity
Relationship
There is a relationship between the luminosity
of a star and its mass
L = M3.5
Where L is luminosity, M is mass in solar units
and applies to all main sequence stars
The power (3.5) can be any value between 3
and 4 as it is itself mass dependant.

16. It is reasonable to assume that the age of the
star is related to its mass. So that luminosity is
determined by energy release/time, and that
energy released is related to mass so,
L α M/T
So, combining the two relations gives
T α M-2.5

17. Question
If a star is twice the mass of the sun, estimate its
lifetime as a main sequence star. Given the
lifetime of the sun is 9.4 x 109
years.

18. Question-solution
If a star is twice the mass of the sun, estimate its
lifetime as a main sequence star. Given the
lifetime of the sun is 9.4 x 109
years.
(Ms/M)2.5
= T/Ts
T = (½)2.5
x 9.4 x 109
= 1.7 x 109

19. The Supergiants

21. The Supergiants
Iron cannot undergo fusion due to its very high
coulombic repulsion (26 protons).It would need
astronomical temperatures. The star has reached a
critical state.
The star will once again begin to collapse into the
core. But no more fusion will take place to
counteract the gravitational collapse.
Incredibly high temperatures lead to the combining
of electrons and protons.

22. The Supergiants
Neutrons and neutrinos are formed in large
quantities.
High energy neutrinos form an outward pressure
wave.
This wave hurtles outward. This shock wave rips
the outer layers off of the star.
The inner core is now exposed.
Huge amount of radiation floods into space.

23. Supernova
The star has become a Supernova.
The luminosity for a brief moment is greater than
the whole luminosity of a galaxy (1 billion stars)!
96% of the stars mass is lost to space.

26. Mass determines star path

27. Neutron capture
There are two forms of neutron capture that can
facilitate stars fusing elements above Fe.
S-process (slow capture): formation of copper,
silver, gold and lead upto to bismuth209
R-process (rapid capture): elements above
bismuth 209 occur only in a supernova explosion.
A large flux of energetic neutrons is produced
and nuclei bombarded by these neutrons build up
mass one unit at a time.

28. Type Ia and IIa supernova

29. Type Ia
The observed spectra over time show sharp maxima and
then die away smoothly and gradually.
The initiation is the detonation of a carbon white dwarf when
it collapses under the pressure of electron degeneracy. It is
assumed that the white dwarf exceeds the Chandrasekhar
limit of 1.4 solar masses.
The spectra of Type I supernovae are hydrogen poor is
consistent with this model, since the white dwarf has almost
no hydrogen.
They can be used as standard candles.

31. Type IIa
Are implosion-explosion events of a massive star.
They have a characteristic plateau in the observed
spectra over time, a few months after initiation. It
is assumed that the energy comes from the
expansion and cooling of the star's outer envelope
as it is blown away into space.
There are strong hydrogen and helium spectra for
the Type II supernovae.

32. Light curves for the supernova

33. Credits
Hyperphysics, http://hyperphysics.phy-astr.gsu.edu/
Windows to the universe
http://www.windows2universe.org/sun/Solar_interior
/Nuclear_Reactions/Fusion/Fusion_in_stars/ncaptur
e.html
Phys 306
http://www.hep.shef.ac.uk/cartwright/phy306/lecture
s.html